This invention relates generally to image projectors, and more particularly, to correcting keystoning problems in projected images.
Portable digital image projectors are common. These projectors, while connected to a PC or VCR, sit on a table and are aimed at a projection surface to show a xe2x80x9cslidexe2x80x9d presentation or a video. Many of these projectors use transmission LCDs, and typically only have a single main lens. The projectors can display images one at the time or as a sequence of images.
These projectors are typically designed so that level undistorted images are projected on the projection surface when the projector is placed horizontally on a level support surface, so that the projector""s optical axis is lined up perpendicular to the, typically, vertical projection surface. If any of the above assumptions is violated, then the resulting image on the projection surface may not be rectangular and will be, at best, a trapezoid, and at worst an arbitrary quadrilateral. This problem is called keystoning.
With prior art projectors, the only way to correct keystoning is to tediously adjust the physical position of the projector by moving it around, tilting and rotating it, until a near rectangular image is displayed. In some cases, it may be impossible to physically adjust the position of the projector. For example, the projector may need to be well above or below the display surface. Some prior art projectors correct the distorted image optically or by the user providing projector positional data.
U.S. Pat. No. 5,548,357, xe2x80x9cKeystoning and focus correction for an overhead projector,xe2x80x9d issued to Appel et. al. on Aug. 18, 1998, describes a system where a test slide is displayed. A user identifies line pairs most parallel to each other. The line pair identification activates a distortion correction program that uses the oblique angle between the horizontal plane through the projector and the viewing screen.
U.S. Pat. No. 5,795,046, xe2x80x9cMethod for pre-compensating an asymmetrical picture in a projection system for displaying a picture,xe2x80x9d issued to Woo on August 1998, describes a system where the projection angle, and the trapezoidal error, is compensated for by the user entering in positional information into the system via a keyboard.
A method corrects keystoning in a projector arbitrarily oriented with respect to a display surface. An elevation angle, a roll angle, and an azimuth angle of an optical axis of the projector are measured with respect to the display surface.
A planar projective transformation matrix is determined from the elevation, roll, and azimuth angles. A source image to be projected by the projector is warped according to the planar projective transformation, and then projected onto the display surface.